ULTRAVIOLET RADIATION AND THE EYE - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 252

diseases vary geographically; however, the lack of consistent epidemiological results is
almost certainly the result of either incomplete or erroneous estimates of outdoor UVR
exposure dose 2,3,5,13-15 . Geometrical factors dominate the determination of UVR exposure of
the eye. The degree of lid opening limits ocular exposure to rays entering at angles near the
horizon. Clouds redistribute overhead UVR to the horizon sky. Mountains, trees and
building shield the eye from direct sky exposure. Most ground surfaces reflect only a small
fraction of incident UVR. The result is that the highest UVR exposure appears to occur
during light overcast where the horizon is visible and ground surface reflection is high. By
contrast, exposure in a high mountain valley (lower ambient temperature) with green
foliage results in a much lower ocular dose. Other findings of these studies show that retinal
exposure to light and UVR in daylight occurs largely in the superior retina 14 .
3. Epidemiological evidence
So why is it important to study the spectral and geometrical exposure of the
human eye from solar UVR? It has long been suggested that the great latitudinal
variation of "the world's most blinding disease," cataract 9 , is strongly suggestive (as in
the case of skin cancers) to result from UVR. The World Health Organization (WHO)
estimates that the worldwide prevalence of cataract exceeds 50 million 9 . Indeed, if this
dependence is examined along with a wide variety of laboratory studies, it suggests that
environmental factors surely plays a major role in the time of onset of lenticular
opacities. However, most epidemiologic evidence points to UVR in sunlight as a
significant risk factor only in one type of cataract, i.e., cortical cataract 12 . The evidence
for UVR as an etiological factor in at least two adverse changes in the cornea, i.e.,
droplet keratopathies and pterygium is much stronger 9,16 . Furthermore, the
environmental studies of Sasaki 10 have provided very important insights into the
geographical variations in the incidence of cataract by examining the different types of
cataract characteristic of different latitudes 10 . Nuclear cataract was shown to be more
common in the tropics; cortical cataracts, more common in mid-latitudes, and posterior
sub-capsular cataracts were not so clearly related to latitude. Despite this latitudinal
variation, some other published epidemiological studies do not appear to show a relation
between UVR and cataract. Nevertheless, a wide variety of scientific evidence, from
laboratory studies of the UV photochemistry of lens proteins to a number of different
animal exposure studies all provide support for the hypothesis that UVR should play a
far greater role in cataractogenesis. Most age-related changes in the skin (from
accelerated aging to skin cancer) have been conclusively shown to result from excessive
exposure to solar UVR (or "sunlight exposure"). Although no one questions that UVR
exposure produces the acute effects of “sunburn” (erythema) and snowblindness
(photokeratitis), some have questioned whether pterygium and droplet keratopathies are
clearly related to UVR exposure 7,9 . Even more under debate are theories that suggest
that UVR and light may affect retinal diseases such as age-related macular degeneration
(AMD) 9 . A better resolution of these questions requires far better ocular dosimetry.
With the strong geographical variations in the incidence of nuclear cataract 10 , it is
surprising that most epidemiological studies show only weak or no apparent relation
between UV or sunlight exposure and the incidence of nuclear cataract 12 . Perhaps it is
necessary to consider the other important environmental parameter-environmental